1. Field of the Invention
The present invention relates to a GaN semiconductor laser having a coating film formed on a front end surface through which laser light is emitted and, more particularly, to a semiconductor laser in which the reflectance of a coating film is set in the range from 3 to 13%, and which is capable of preventing separation of the coating film and being reliable.
2. Background Art
Semiconductor lasers are being widely used in optical disk systems, optical communication, etc. In recent years, GaN semiconductor lasers which emit blue laser light have been put to use. A semiconductor laser has a front end surface through which laser light is emitted and a rear end surface opposed to the front end surface. Coating films are formed on the front end surface and the rear end surface to achieve, for example, a reduction in the operating current for the semiconductor laser, prevention of return light and an increase in output.
A semiconductor laser required to have an increased output ordinarily has a coating film of a low reflectance formed on the front end surface and a coating film of a high reflectance formed on the rear end surface. The reflectance of the coating film on the rear end surface is ordinarily 60% or more, preferably 80% or more. On the other hand, it is not sufficient to simply lower the reflectance of the coating film on the front end surface. The reflectance at the front end surface is set according to a characteristic required of the semiconductor laser. For example, a reflectance of about 0.01 to 3% is set in a fiber amplifier excitation semiconductor laser used with a fiber grating; a reflectance of about 3 to 7% in an ordinary high-output semiconductor laser; and a reflectance of about 7 to 13% in a case where there is a need to take a measure against return light.
FIG. 3 is a diagram showing a film thickness dependence of the reflectance of a coating film using a single layer of Al2O3 film. For example, in a case where the film thickness of Al2O3 film is set to 91.5 nm in order to set the reflectance to about 10%, the actual reflectance is 9.91%; the reflectance can be set within the target range from 3 to 13%. In this case, if the film thickness of Al2O3 film varies by ±5%, the reflectance varies largely between the minimum 7.72 and the maximum 12.03%. To limit this reflectance variation, the film thickness may be set in the vicinity of an inflection point of the reflectance. In the case of Al2O3 film, however, the reflectance at the inflection point is 1% or less, out of the range from 3 to 13%.
FIG. 4 is a diagram showing a film thickness dependence of the reflectance of a coating film using a single layer of Ta2O5 film. In the case of Ta2O5 film, the reflectance at an inflection point is about 10%. Therefore, if a single layer of Ta2O5 film is used as coating film on the front end surface, the reflectance can be set within the range from 3 to 13% while limiting variation in the reflectance.
A technique using a two-layer film formed of AlOx film (0<x<1.5) and Al2O3 as coating film has also been proposed (see, for example, Japanese Patent Laid-Open No. 2002-335053).